The present invention relates generally to an exposure method and apparatus used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as fine contact hole patterns used for micromechanics, and more particularly to a so-called immersion type exposure method and apparatus that immerse, in liquid, a surface of an object to be exposed, and a bottom surface of a projection optical system closest to the object, and expose an object via the liquid.
Reduction projection exposure apparatus has been conventionally employed which use a projection optical system for projecting a circuit pattern formed on a mask (reticle) onto a wafer, etc. and for transferring the circuit pattern, in manufacturing such fine semiconductor devices as semiconductor memories and logic circuits in the photolithography technology.
The critical dimension transferable by the projection exposure apparatus is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Smaller resolution has been demanded with a demand for finer semiconductor devices. The exposure light is requested to move to small wavelength, and the projection optical system is expected to improve resolution using higher NA. Because of the difficulty of changing the exposure wavelength, a projection optical system has accelerated an improvement of its NA; for example, a projection optical system having NA=0.9 has been developed.
On the other hand, light sources for the exposure apparatus have changed from a KrF laser (with a wavelength of 248 nm) to an ArF laser (with a wavelength of 193 nm). At present, an F2 laser (with a wavelength of 157 nm) and EUV (with a wavelength of 13.5 nm) have been developed as next generation light sources.
In such a situation, immersion exposure has called attentions, as disclosed in Japanese Patent Publication No. 10-303114, as a method that uses ArF laser (with a wavelength of 193 nm) and F2 laser (with a wavelength of 157 nm) for improved resolution. The immersion exposure uses a liquid as a medium at a wafer side. It fills the space between the projection optical system and a wafer, to promote a higher NA. Specifically, the projection optical system has a numerical aperture (“NA”) of n·sin θ, where “n” is a refractive index of the liquid, and NA can increase up to “n”.
Influence of polarized light on imaging performance becomes non-negligible as NA becomes higher and higher, because the imaging performance becomes different according to polarization directions as the light has a larger incident angle upon a wafer.
The performance for two-beam imaging is much more affected by polarization than that for three-beam imaging. In the three-beam imaging that forms an image through interference among three beams, i.e., the 0th order beam and the ±1st order diffracted beams, an angle does not reach 90° between the 0th order beam and the 1st order diffracted beam and between the 0th order beam and the −1st order diffracted beam, which form a basic frequency for imaging, and the influence of polarization does not appear significantly. On the other hand, two-beam imaging includes the interference between two 1st diffracted beams and that between the 0th order light and one of the ±1st order diffracted beams. The influence of polarization on the imaging performance is serious because two beams that form the basic frequency have large angles.
Moreover, in the immersion case, two-beam imaging can problematically meet a condition in that no image is formed at all in a certain polarized light direction. This phenomenon has not occurred in a conventional non-immersion optical system. When two interfering beams are supposed to form an image as shown in FIG. 16A, p-polarized beams with a polarization direction on the paper surface do not interfere with each other or do not contribute to imaging because they form an angle of 90°. When two beams are in symmetry and form an angle of 90°, an incident angle is 45° and sin 45°=0.7.
On the other hand, s-polarized beams shown in FIG. 16B have a polarization direction orthogonal to the paper surface and form an image with good contrast. This fact now defines a polarization direction that forms an image with good contrast as an s-polarized component or s-polarized light. As discussed, the s-polarized light has a polarization direction orthogonal to the paper surface when two beams interfere with each other on the paper surface. The polarization direction has implications with a direction in which a pattern is formed. When an interference fringe is formed by two beams, a direction of s-polarized light accords with each longitudinal direction of an interference fringe formed by two beams: An X direction is a direction of the s-polarized light in forming an interference pattern whose fine structure extends in the X direction. A Y direction is a direction of the s-polarized light in forming an interference pattern whose fine structure extends in the Y direction.
When two-beam interference is considered which forms an image through interference between two beams having angles of ±θr in resist, where no is a refractive index of a medium between a projection optical system and the wafer, θo is an angle in the medium, and nr is a refractive index of the resist, Equation (1) below is established from the Snell's law:nr·sin θr=no·sin θo  (1)
When the medium is the air, Equation (2) below is established since no=1 and sin θo<1:nr·sin θr=no·sin θo<1  (2)
In the case of ArF excimer laser, the resist typically has a refractive index nr=1.7, and Equation (2) leads to sin θr<0.59. Thus, when the medium is the air, the angle θr in the resist never shows sin θr=0.7. The cross angle of 90° condition in resist, consequently, never occurs.
On the other hand, in the case of immersion where the medium is liquid, Equation (3) below is established. When the medium has a refractive index no=1.47:nr·sin θr=no·sin θo<1.47  (3)
Since the resist usually has the refractive index nr=1.7, sin θr<0.86. Thus, when the medium is liquid, θr can attain the condition sin θr<0.7.
Again, a condition never meets sin θr<0.7 when the medium is the air, whereas a condition can meet sin θr<0.7 when the medium is liquid, whereby the p-polarized light does not interfere and the contrast from the p-polarized light becomes zero. When the illumination light is non-polarized, only the s-polarized light, which is the half of the incident light, contributes to imaging. The p-polized light does not contribute to the imaging, thus halving the contrast, and creating a reduced contrast problem.
For example, in the case of ArF excimer laser, Equation (4) below is established when the medium is water and no=1.47:
                              sin          ⁢                                          ⁢                      θ            o                          =                                                            n                r                                            n                o                                      ⁢            sin            ⁢                                                  ⁢                          θ              r                                =                                                    1.7                1.47                            ⁢              sin              ⁢                                                          ⁢              45              ⁢              °                        =            0.81                                              (        4        )            
As a consequence, water as the medium causes a condition that provides no interference of p-polarized light when sin θo=0.81. Thus, the p-polarized light does not form an image when an incident angle in the medium approximately meets sin θo=0.8. This problem is inevitable since an optical system for immersion is required to have super-high NA, which means much larger than 1.0. In this case, the angle in the medium meets sin θo=0.8. Even in case of F2 excimer laser, the similar relationship is established when sin θo is approximately 0.8 since the medium has a refractive index of around 1.36 and the resist has a refractive index of 1.5 or higher.
It is known that increased refractive indices of the resist and liquid are effective to increase NA. The resist and liquid have different refractive indices according to their materials. As disclosed in U.S. Pat. No. 4,346,164, a small difference of refractive index between the resist and liquid is preferable, but, in the usual case, sin θr in the resist is slightly smaller than sin θo in the medium. The inventors have discovered that since it is anticipated that the resist for exclusive use with the immersion will be developed, it is preferable to set sin θo in the medium in the exposure apparatus side to be approximately equivalent with sin θr in the resist. Thus, the condition that provides no interference of p-polarized light may be regarded as sin θo≈sin θr=0.7.
As discussed, the high NA projection optical system is necessary for finer patterns, while the imaging performance deteriorates due to the p-polarization features of the high NA imaging beam. In some cases, the desired pattern cannot be formed as is predicted by the simple scalar theory, which does not count the polarization effect.